A number of metals including aluminum, lead, magnesium, zinc, zirconium, titanium, and silicon can be produced by electrolysis processes. Each of these electrolytic processes preferably employs an electrode having a hollow interior.
One example of an electrolysis process for metal production is the well-known Hall-Heroult process producing aluminum in which alumina disso fluoride bath is electrolyzed at temperatures of about 960° C.-1000° C. As generally practiced today, the process relies upon carbon as an anode to reduce alumina to molten aluminum. Despite the common usage of carbon as an electrode material in practicing the process, there are a number of serious disadvantages to its use, and so, attempts are being made to replace them with inert anode electrodes made of for example a ceramic or metal-ceramic “cermet” material.
Ceramic and cermet electrodes are inert non-consumable and dimensionally stable under cell operating conditions. Replacement of carbon anodes with inert anodes allows a highly productive cell design to be utilized, thereby reducing costs. Significant environmental benefits are achievable because inert electrodes produce essentially no CO2 or fluorocarbon or hydrocarbon emissions. Some examples of inert anode compositions are found in U.S. Pat. Nos. 4,374,761; 5,279,715; and 6,126,799, all assigned to Alcoa Inc.
Although ceramic and cermet electrodes are capable of producing aluminum having an acceptably low impurity content, they are relatively expensive. Also, to save costs most have a hollow interior into which a conductor rod is sintered/sealed in place. These inert anodes are molded, extruded, or preferably isostatically pressed usually at about 30,000 psi around a mandrel, to provide an unsintered green anode, which must be subsequently fired to sinter it. In order to fire it the anode must be placed upside down on a sintering tray. This provides a variety of problems.
In the past, a solid cylindrical mandrel and accompanying flexible mold were used to consolidate ceramic/cermet material into a hollow anode shape through isostatic pressing. After pressing, the mandrel was removed from the anode shape and the shape removed from the mold. The unfired green part was then gripped by a variety of devices and placed upside down (hollow side down) on a firing tray for sintering. After sintering in a kiln, the assembly of an anode was completed. This concept required the use of multiple handling devices.
Sacrificial, extruded, metal anodes have been made with threaded ends machined into their top exterior for insertion into hot water heaters as taught in U.S. Pat. No. 5,728,275 (Twigg). Here, the anode itself is a metal more electropositive than a metal surface to be protected. What is needed here is a metal mandrel for forming ceramic or ceramic metal electrodes where the mandrel is easily inserted, removed and performs multiple functions to allow ease of producing the electrode. It is a main object to provide a new mandrel system and inexpensive process for forming green inert anodes. One example of an inert anode assembly for an aluminum smelting cell is shown in FIG. 3 of U.S. Patent Application Publication 2001/0035344 A1 (D'Astolfo Jr. et al.) where cup shaped anodes are used.